If you are doing this to save a device for light use (MP3 player, clock), the Custom ROM route is your best bet. If you are looking for a daily driver, it is highly recommended to look for a device running at least Android 10 or newer.
Contrary to popular belief, Android 5.0 Lollipop introduced – a battery-saving feature. On older hardware, a well-optimized patched Lollipop build runs smoother than KitKat. update android 44 4 to 50 download patched